fix(aibox): batch H audit fixes - subagent cancel propagation, scheduler timeout, exec process-group reaping

Fourth audit round (see docs/reports/aibox_audit_20260817.md §11):
- H1: shared parent cancel event no longer cleared on subagent first think
  (N5 regression); parent-already-cancelled skips subagent start
- M1: /reload_prompts moved off event loop (run_in_executor, N3 sibling)
- M2: skill --help scan prewarmed before serving connections
- M3: scheduler task-level timeout (scheduler.task_timeout, default 600s)
- M4: get_recent_memories(None) capped instead of full-table load
- M5: memo delete now physically removes rows (was complete-only)
- M6: PromptManager.reload builds new list then atomically swaps
- L1-L12: read_file arg coercion, proxy save failure reported, SSE
  utf-8 tolerance, persona realpath check, truncation char boundary,
  compressor div0, skill path quoting, ask_user required, POSIX
  process-group kill (pgid captured at Popen), web scheme whitelist,
  registry redact substring match, Bearer redaction in failover logs

aibox/subsystems/conversation_compressor.py

@@ -514,7 +514,8 @@ class ConversationCompressor:
         final_tokens = self.estimate_tokens([m for m in final_messages if m.get("role") != "system"])
 
         print(f"\n✅ {i18n.get('conversation_compressed', original_count, final_count)}")
-        print(f"   Token: {original_tokens} -> {final_tokens} (压缩率: {final_tokens/original_tokens:.1%})")
+        # L6：压缩块全空时 original_tokens 可能为 0，除零防护
+        print(f"   Token: {original_tokens} -> {final_tokens} (压缩率: {final_tokens / max(original_tokens, 1):.1%})")
 
         # 检查是否压缩到阈值以下
         if final_count >= threshold: